The present invention relates to a sheet delivery mechanism, and more particularly to a sheet delivery mechanism suitable for an image forming apparatus such as a copying machine, a facsimile or a page printer, and a sheet treatment apparatus such as an automated hole punch for treating a sheet.
The sheet delivery mechanism of this type is always provided in a sheet treatment apparatus and delivers a sheet correspondingly to sheet treatment operation.
For example, an electrophotographic image forming apparatus has a structure in which a document image is exposed onto a photoreceptor to form an electrostatic latent image, the electrostatic latent image is developed into a toner image, and the toner image is transferred onto a sheet so as to obtain copy. Such an electrophotographic image forming apparatus comprises a pair of delivery rollers for delivering a sheet to a photoreceptor, and a resist device for delivering the sheet at a timing corresponding to an image position on the photoreceptor after delivery is started by the delivery rollers. The resist device has the very important function of delivering the sheet at a predetermined timing so as to move the sheet into the transferring position of the document image on the photoreceptor.
Referring to a timing control method of the sheet delivery mechanism described above, the phase of a rotator which rotates synchronously with the photoreceptor is detected by a microswitch, and a solenoid of a clutch of each sheet delivery means is operated according to a signal generated by the microswitch.
According to the above-mentioned method, however, there is variation in response time until a signal is generated by the microswitch and the solenoid is actually operated. Consequently, it cannot be expected that sheet delivery operation is carried out at an accurate timing. In case of a thin sheet, wrinkles may occur due to the pressure welding force of the delivery rollers. In case of a thick sheet, delivery may not be carried out in relation to an angle of insertion of nip members of the delivery rollers.
To cope with the above-mentioned drawbacks, there has been proposed the prior art shown in FIG. 26 (Japanese Patent Publication No. 7462/1990).
According to the prior art, a stopper 1032 is provided on the downstream side of sheet delivery rollers 1007a and 1007b. The stopper 1032 halts a sheet in a first sheet delivery state (shown by a solid line in FIG. 26) and releases a sheet halt in a second sheet delivery state (shown by a two-dotted line 1032a in FIG. 26).
A link mechanism R is provided between a document table 1001 and the stopper 1032 in order to transfer the stopper 1032 from the first sheet delivery state to the second sheet delivery state.
The link mechanism R includes a pair of cams 1019b and 1019c, a projection for drive 1020, a first arm 1022, a first pivot 1023, a second arm 1028, a second pivot 1029, a pair of forks 1031a and 1031b, and a tension spring 1033. The cams 1019b and 1019c are provided in a bottom trench 1019a on the document table 1001. The projection 1020 is driven by the cams 1019b and 1019c. The first arm 1022 interlocks with the projection 1020. The first pivot 1023 pivots the first arm 1022. The second arm 1028 interlocks with the first arm 1022 when the first arm 1022 is driven around the first pivot 1023. The second pivot 1029 pivots the second arm 1028. The forks 1031a and 1031b are integrally formed with the second arm 1028 and causes the stopper 1032 to fluctuate when the second arm 1028 is driven around the second pivot 1029. The tension spring 1033 urges the projection 1020 upward through the first arm 1022.
In FIG. 26, when a main switch is turned on, the document table 1001 moves to a document exposure position. When a copy button is pressed, the document table 1001 starts to move from a home position to expose a document.
In the first sheet delivery state, a sheet is delivered by the delivery rollers 1007a and 1007b, and the tip of the sheet comes in contact with the stopper 1032 and is aligned in parallel with an axis of a photoreceptor drum 1003. In this state, upper and lower rollers 1007a and 1007b slip on the sheet.
When the cams 1019b and 1019c press down the projection 1020 at a predetermined timing, the first and second arms 1022 and 1028 rotate around the corresponding pivots 1023 and 1029 according to the operation of the projection 1020. As a result, an apparatus shown in FIG. 26 is transferred from the first sheet delivery state to the second sheet delivery state. Thereby, the forks 1031a and 1031b of the second arm 1028 fall to lower the stopper 1032. Consequently, a delivery passage 1036 is opened. When the delivery passage 1036 is opened, the sheet is delivered to the photoreceptor drum 1003 by the sheet delivery rollers 1007a and 1007b again. When the rear end of the sheet passes through the stopper 1032 and the projection 1020 then moves from the cams 1019b and 1019c to the bottom trench 1019a by the urging force of the tension spring 1033, the stopper 1032 is also driven upward through the link mechanism R. Consequently, the delivery passage 1036 is closed again, so that the apparatus returns to the first sheet delivery state.
Referring to the sheet delivery mechanism according to the prior art, it is required that the delivery rollers 1007a and 1007b slip on the sheet in order to stop sheet delivery when the sheet comes in contact with the stopper 1032 in the first delivery state. Consequently, the pressure welding force between the delivery rollers 1007a and 1007b cannot be made great. For this reason, the sheet is unstably gripped by the delivery rollers 1007a and 1007b so that poor delivery may easily occur, for example, the sheet is obliquely supplied to the photoreceptor drum 1003. Thus, it is required that a pair of auxiliary delivery rollers having greater pressure welding force are further provided between the stopper 1032 and the photoreceptor drum 1003. As a result, production costs are increased.
As disclosed in the above-mentioned prior art, the start position of the sheet is controlled by the stopper 1032. Consequently, when there is variation in sheet delivery state precision of the stopper 1032 and delivery rollers 1007a and 1007b, and that of the stopper 1032 and photoreceptor drum 1003, the start position of the sheet for the photoreceptor drum 1003 varies. For this reason, it is impossible to deliver the sheet at a timing corresponding to an image position on the photoreceptor drum 1003. As a result, an image tip position on the sheet is shifted so that an image is made poor.
In the case where the apparatus is stopped due to clogging or the like, the delivery rollers 1007a and 1007b cannot synchronize with the photoreceptor drum 1003 when the apparatus is reactivated without returning the document table 1001 to the home position and the sheet is inserted. Consequently, a timing deviates so that copy failure may be caused.
In consideration of the above-mentioned problems, it is an object of the present invention to provide a sheet delivery mechanism having a simple structure which can be adopted in a sheet treatment apparatus, capable of causing a sheet to smoothly stand by when the sheet to be delivered is halted, and of surely delivering the sheet when a sheet halt is released for delivery.
It is another object of the present invention to provide a sheet delivery mechanism capable of preventing a sheet from being delivered so as not to mistreat the sheet even if the sheet is inserted when a sheet treatment apparatus is reactivated in a state where its working member does not return to an idling condition.
It is yet another object of the present invention to provide a sheet delivery mechanism which does not need a pair of auxiliary delivery rollers between a stopper and a photoreceptor and can reduce production costs in the case where the sheet treatment apparatus is an image forming apparatus.
It is a further object of the present invention to provide a sheet delivery mechanism capable of holding a sheet with high precision by preventing the sheet delivery state relationship between a sheet halting member and a photoreceptor from varying and of delivering the sheet to the photoreceptor at a predetermined timing so that an image tip position on the sheet can be prevented from being shifted in the case where the sheet treatment apparatus is an image forming apparatus.